The invention relates to a method to avoid interference between recurring alignment and overlay marks formed when a photomask is reused at different vertical heights in fabrication of an integrated circuit.
In conventional integrated circuit design, it is not usual to reuse a photomask. In some complex structures, however, it may be most cost-effective to use the same photomask more than once. When a photomask is reused, the reference marks (alignment and overlay marks) used to align the following photomask and to check the alignment achieved are reproduced in almost exactly the same location as in the first use of the mask. The prior reference marks can interfere with the present reference marks.
There is a need, therefore, for a way to reuse photomasks while preventing interference between recurring reference marks.